Multiplex transmission apparatuses, multiplex transmission networks, and operation method therefor

ABSTRACT

An overhead passing processing circuit in a synchronous multiplex network, with a circuit for extracting predetermined administration and maintenance operating information from multiplexed signals, a crossconnecting circuit for crossconnecting information extracted by the extraction circuit stage, and an insertion circuit for inserting an output of the crossconnecting circuit into a predetermined location in the overhead of the multiplexed signal to be transmitted.

RELATED APPLICATIONS

This application is a continuation application of U.S. patentapplication Ser. No. 09/731,424, filed on Dec. 6, 2000, now U.S. Pat.No. 6,697,386, which is in turn a continuation of U.S. application No.08/863,675, filed May 27, 1997, now U.S. Pat. No. 6,169,754, issued onJan. 1, 2001, the entirety of which are incorporated herein byreference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to configurations of digital transmissionapparatuses and configurations of communication networks using thedigital transmission apparatuses, and more particularly toconfigurations of transmission apparatuses for use in a synchronousdigital hierarchy and configurations of communication networks using themultiplex transmission apparatuses.

2. Description of the Related Art

In today's digital transmission networks, the technology ofsynchronization has been advanced to such a degree that a communicationnetwork is synchronized with faster transmission apparatuses employingoptical transmission. For functions and configurations of the digitaltransmission networks and transmission apparatuses, worldwide standardshave been established such that a transmission apparatus and/or acommunication network may be introduced in conformity to the standardsto provide high quality transmission anywhere in the world. Examples ofspecific standards may include the standard (established in 1988) on atransmission system referred to as “SDH” (Synchronous Digital Hierarchy)defined in Recommendation G. 707 and so on by InternationalTelecommunication Union (hereinafter abbreviated as “ITU-T), and thestandard (established in 1991) on a transmission system referred to as“SONET” (Synchronous Optical Network) defined in Standard T1.105 byAmerican National Standard Institute (hereinafter abbreviated as“ANSI”), both of which define the configuration of optical synchronouscommunication systems and functions of transmission apparatuses.

Both SDH and SONET define the processing (transmission,multiplexing/demultiplexing, and so on) of a synchronous multiplexedsignal (frame) which comprises a main signal portion referred to as a“Payload” in which digitized main signals are multiplexed, and signalsreferred to as “overheads”, added to the payload, for administration andmaintenance operations for a transmission apparatus and communicationnetwork. The overheads include pointers which are used to perform stuffcontrols such as frame phase synchronization and frequency adjustment toprovide a transmission system which has a less transmission delay and ahigher administration and maintenance operation performance thanconventional digital synchronous transmission apparatuses. The overheadsadded to the frame are classified into a section overhead (SOH) and aline overhead (LOH). The section overhead is used for administration andmaintenance operations for each transmission span between transmissionapparatuses and regenerators (defined as a section), and generated in anapparatus (including a regenerator), transmitted through a transmissionspan, and terminated at a next apparatus. The line overhead is used foradministration and maintenance operations for each transmission intervalbetween transmission apparatuses which process multiplexed main signals(defined as a line). The line overhead is generated in a transmissionapparatus, transmitted through transmission spans and regenerators, andterminated at a next multiplexing apparatus. Examples of transmissionapparatuses and networks adopting the above-mentioned SDH or SONET aredescribed in JP-A-4-79628 and JP-A-5-114892.

In a transmission network in conformity to SDH or SONET, multiplexersmay be occasionally connected to each other on a transmission linethrough regenerators. For carrying out the administration andmaintenance operations between the multiplexers in such a transmissionnetwork, the line overhead is used to transmit and receive data andspeech signals necessary to the administration and maintenanceoperations between the multiplexers. For example, D-bytes referred to as“data communication channels” and E-bytes referred to as “orderwires” ofthe line overhead are used to transmit and receive such data and speechsignals between the multiplexers. Specifically, when a multiplexer onthe transmission side inserts data and speech information into D4-D12bytes and E2 byte of a line overhead and transmits the line overheadonto a transmission line, the line overhead is terminated at adestination multiplexer through the transmission line and regenerators,thereby carrying out the administration and maintenance operationsbetween the multiplexers.

As the number of subscribers increases in the transmission network or asan increased amount of signals is communicated through the transmissionnetwork, extension and/or modifications in the transmission network,such as installation of additional multiplexers and replacement to thetransmission network to a faster transmission line, may be required forsupporting the increase in subscribers and the amount of communicatedsignals. For example, if the amount of communications increases, theexisting transmission network is reconfigured, wherein similarmultiplexers to those so far used are additionally installed in thetransmission network, and a plurality of faster and larger-scaledmultiplexers are introduced for processing signals in place ofregenerators to modify the transmission network so that the multiplexersare connected through faster transmission line in the transmissionnetwork.

However, the reconfiguration of the transmission networks as mentionedabove results in a problem involved in SDH and SONET standards.Specifically, since the line overhead is terminated at each transmitteraccording to SDH and SONET standards, an overhead outputted from anexisting multiplexer may be terminated at an additionally installedfaster multiplexer, so that the overhead is not transmitted to a far-endmultiplexer which has so far received this overhead. In other words, thereconfiguration results in a lack of the administration and maintenanceoperations previously performed between the multiplexers before thereconfiguration. Thus, while the transmission capability of thetransmission network for transmitting main signals is improved by thereconfiguration of the transmission network, the reconfiguration causeschanges in the administration and maintenance operation capability ofthe transmission network, such as lack of the administration andmaintenance operations so far performed between transmissionapparatuses. Since administration and maintenance operation informationpreviously provided is no longer available to a craft person dedicatedto the maintenance of the transmission network, the craft person maysuffer from quite inconvenient situations.

SUMMARY OF THE INVENTION

It is an object of the present invention to prevent changes inadministration and maintenance operation capability due to areconfiguration of a transmission network as mentioned above, andspecifically to provide a transmission apparatus and a transmissionnetwork having an administration and maintenance operation capabilitywhich is not affected by any modification to the transmission network orwhich enable more flexible and high performance administration andmaintenance operations in a simple configuration.

More specifically, the present invention provides a transmissionapparatus which additionally has a function of passing through anoverhead instead of processing the overhead standardized by SDH andSONET. The present invention also provides a transmission network, whichis flexible and superior in administration and maintenance operationperformance, wherein the transmission network uses transmissionapparatuses as mentioned above, such that arbitrary transmissionapparatuses in the transmission network are permitted to transmit andreceive an arbitrary overhead therebetween. The present invention alsoprovides a method of operating the configuration for the transmissionnetwork.

In a more detailed aspect, the present invention provides circuits andapparatus for use in a digital transmission apparatus, in a simplestructure, for selectively cross-connecting a received tributaryoverhead and transmitting the cross-connected overhead to a far-endtransmission apparatus. The present invention also allows for atributary overhead containing information which cannot be interpreted ata far-end destination apparatus simply by passing the overhead throughintervening apparatuses, and provides circuits and apparatus, in asimple structure, for converting a tributary overhead into informationusable by a far-end destination apparatus and for transmitting theconverted information to the far-end destination apparatus. Inparticular, the present invention provides simple circuits and apparatusfor accurately notifying the number of transmission errors, if any, in atributary through which an overhead is transmitted and received, toenable the management of transmission quality.

Moreover, the present invention provides a method of selectivelycross-connecting an overhead and transmitting and receiving thecross-connected overhead between selected multiplexers, and a method ofdetecting and notifying transmission errors which have occurred in atributary.

To solve the problems mentioned above, a multiplex transmissionapparatus according to the present invention receives a multiplexedtributary signal comprising a payload having a plurality of main signalsmultiplexed therein and overhead bytes including a plurality ofmaintenance information associated with administration and maintenanceoperations, performs termination processing for the administration andmaintenance operation information and transmission processing for thepayload, thereafter converts the multiplexed tributary signal into amultiplexed high-speed signal comprising a payload including mainsignals which have been processed for transmission and a plurality ofadministration and maintenance operation information, and transmits themultiplexed high-speed signal. The multiplex transmission apparatuscomprises a circuit or a apparatus for selecting predeterminedmaintenance information from the plurality of maintenance informationincluded in the received multiplexed tributary signal, and inserting thepredetermined maintenance information into an overhead byte in themultiplexed high-speed signal on the high-speed transmission side tothereby pass the predetermined maintenance information through themultiplex transmission apparatus.

Specifically, the multiplex transmission apparatus comprises an overheadpassing circuit or a passing apparatus for passing maintenanceinformation which is composed of an extraction circuit or an extractionapparatus for extracting predetermined maintenance information from theplurality of maintenance information included in the receivedmultiplexed tributary signal, and an insertion circuit or an insertionapparatus for inserting extracted maintenance information into apredetermined location in the overhead bytes of the multiplexedhigh-speed signal to be transmitted. The overhead passing circuit orapparatus may be additionally provided with an cross-connecting circuitor cross-connecting apparatus for cross-connecting extracted maintenanceinformation.

Also, the multiplex transmission apparatus receives a plurality ofmultiplexed tributary signals each comprising a payload and overheadbytes including a plurality of maintenance information associated withadministration and maintenance operations, performs terminationprocessing for the plurality of maintenance information and multiplexingof the plurality of payloads in a payload having a larger multiplexingdegree, converts the plurality of multiplexed tributary signals into amultiplexed high-speed signal comprising the larger payload and overheadbytes, added to the large payload, having a size larger than theoverhead bytes on the tributary side including a plurality ofmaintenance information associated with administration and maintenanceoperations, and transmits the multiplexed high-speed signal. Themultiplex transmission apparatus comprises an overhead passing circuitor a passing apparatus for passing maintenance information which iscomposed of an extraction circuit or an extraction apparatus forextracting predetermined maintenance information from the plurality oftributary maintenance information, an cross-connecting circuit or across-connecting apparatus for cross-connecting information extracted bythe extraction circuit or apparatus, and an insertion circuit or aninsertion apparatus for inserting an output of the cross-connectingcircuit or apparatus into a predetermined location of the high-speedoverhead bytes, wherein the maintenance information received from theplurality of transmission paths is collectively transferred or passed.

A far-end transmission apparatus connecting this multiplex transmissionapparatus through high-speed transmission line comprises an overheadpassing circuit or passing apparatus which is composed of an extractioncircuit or an extraction apparatus for extracting predeterminedmaintenance information from a plurality of high-speed maintenanceinformation, a cross-connecting circuit or a cross-connecting apparatusfor cross-connecting information extracted by the extraction circuit orapparatus, and an insertion circuit or an insertion apparatus forinserting an output of the cross-connecting circuit or apparatus into apredetermined location of the tributary overhead bytes, wherein uponreceiving high-speed overhead bytes including maintenance informationcollectively transferred thereto, these signals are extracted,cross-connected, and inserted into predetermined locations of aplurality of tributary overhead bytes to be transmitted onto a pluralityof tributary transmission lines.

Here, each of the foregoing multiplex transmission apparatuses processesthe multiplexed signals defined in Recommendation G. 707 ofInternational Telecommunication Union or in Standard T1. 105 of AmericanNational Standard Institute. Maintenance information to be passed isthat included in a section overhead and a line overhead of tributaries.The maintenance information to be passed is transferred in a lineoverhead of a multiplexed high-speed signal. In addition, when thesemultiplexers are directly connected without regenerators, a sectionoverhead is also used to pass or transfer an increased amount ofmaintenance information. When E byte, F byte, D byte, K byte, and Zbyte, defined by the standard, are to be passed as maintenanceinformation, these bytes are selected and passed as they are.

On the other hand, as a configuration for passing information on numberof errors on a transmission path such as B bytes defined by thestandard, a transfer circuit or a transfer apparatus is provided asfollows. The transfer circuit or apparatus, upon detecting the number oferrors which have occurred on a receiving tributary transmission line,inserts this number of occurring errors into an overhead byte of amultiplexed high-speed signal such that the number of errors istransferred in the multiplexed high-speed signal. Then, a far-endmultiplex transmission apparatus which receives transferred maintenanceinformation comprises an extraction circuit or an extraction apparatusfor extracting the number of transmission errors of near-end tributary,an adder circuit or an adding apparatus for adding the extracted numberof transmission errors of the near-end tributary and the number ofhigh-speed transmission errors detected by this multiplex transmissionapparatus, and a circuit or an apparatus for inserting the additionresult into a second high-speed signal overhead and transferring theoverhead, or for adding the number of transmission errors correspondingto the addition result to an error detecting signal of tributary. Foranother configuration or method, a multiplex transmission apparatuslocated in the middle of a high-speed transmission line transfer thenumber of transmission errors in tributary, and a multiplex transmissionapparatus comprises detector circuit or a detecting apparatus fordetecting the number of errors which have occurred in a high-speedtransmission lines, an extraction circuit or an extraction apparatus forextracting the number of transmission errors of the near-end tributary,an adder circuit or an adding apparatus for adding the extracted numberof transmission errors and the number of transmission errors detected bythe detector circuit or apparatus, and a circuit or an apparatus foradding the number of transmission errors corresponding to the additionresult to an error detecting signal of tributary.

Furthermore, to solve the problems mentioned above, a multiplextransmission network according to the present invention employs theapparatus as described above for a multiplex transmission apparatus forpassing arbitrary maintenance information of the tributaries throughintermediate multiplexers between arbitrary multiplex transmissionapparatuses, so that the arbitrary multiplex transmission apparatusestransmit and receive arbitrary maintenance information of thetributaries therebetween.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a frame structure diagram illustrating the structure of aframe of a tributary multiplexed signal (OC-12);

FIG. 2 is a frame structure diagram illustrating the structure of aframe of a high speed multiplexed signal (OC-192);

FIG. 3 is a function explanation diagram for explaining the functions ofoverheads in a multiplexed signal;

FIGS. 4A, 4B and 4C are network configuration diagrams for explainingthe configuration of transmission networks and overhead processingintervals;

FIG. 5 is a block configuration diagram illustrating the configurationof a transmission apparatus according to the present invention;

FIG. 6 is an overhead structure diagram for explaining how tributaryoverheads are passed through a transmission apparatus (multiplexer)according to the present invention;

FIG. 7 is an overhead structure diagram for explaining how tributaryoverheads are passed through a transmission apparatus according to thepresent invention;

FIG. 8 is an operation explanation diagram for indicating a calculationarea of a tributary overhead (B2 bytes) in the transmission apparatusaccording to the present invention;

FIG. 9 is a network configuration diagram illustrating the configurationof a transmission network employing the transmission apparatusesaccording to the present invention;

FIG. 10 is a block configuration diagram illustrating the configurationof another transmission apparatus (ADM) according to the presentinvention;

FIGS. 11A and 11B are network configuration diagrams illustrating theconfiguration of transmission networks employing other transmissionapparatuses according to the present invention;

FIGS. 12A and 12B are operation explanation diagrams for explaining theconfiguration of transmission error detection using a tandem connectionin a transmission network employing the transmission apparatusesaccording to the present invention; and

FIG. 13 is a network configuration diagram illustrating theconfiguration of a transmission network using a tandem connectionemploying the transmission apparatuses according to the presentinvention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Embodiments of transmission apparatuses as well as embodiments oftransmission systems and networks according to the present inventionwill hereinafter be described in detail with reference to theaccompanying drawings.

It should be first noted that the embodiments of the present inventionare described, as an example, with reference to transmission apparatusesand transmission systems mainly used in conformity to SONET. While aconventional transmission system is such that an overhead of atransmission frame is terminated at each apparatus, a transmissionapparatus of the present invention has a function of passing a tributaryoverhead therethrough to enable desired apparatuses within atransmission network to transmit and receive the overhead, therebymaking it possible to improve the administration and maintenanceoperation performance of the transmission apparatus. Transmissionapparatuses and transmission systems using the SDH will also havesubstantially the same configurations.

For facilitating the understanding of the present invention, theconfigurations and operations defined by SONET and SDH are brieflyexplained prior to descriptions on the configurations and operations oftransmission apparatuses and transmission systems according to thepresent invention.

FIGS. 1 and 2, which are frame structure diagrams illustrating thestructures of frames in multiplexed signals defined by SONET, illustratethe configurations of multiplexed signals of OC-12 (622.08 Mb/s) andOC-192 (9953.28 Mb/s). FIG. 3 is an explanatory diagram showingfunctions of respective overhead bytes included in a multiplexed signal.In FIGS. 1-3, signals in columns 1-36 (OC-12) or in columns 1-576(OC-192) constitute an overhead (see FIG. 3 as for functions ofrespective bytes. Undefined bytes are indicated by “x” in FIGS. 1 and2), and the remaining area constitutes a payload portion in which mainsignals are multiplexed.

FIGS. 4A-4C are network configuration diagrams for explaining theconfigurations of transmission networks adopting SDH or SONET andtransmission intervals through which overheads are transmitted. Withineach of the overheads in the multiplexed signals, rows 1-3 constitute aso-called section overhead which is associated with administration andmaintenance operations for each transmission span (defined as a“section”) between transmission apparatuses and/or regenerators. Anoverhead generated in an apparatus (including a regenerator) istransmitted through a transmission span and terminated at a nextapparatus (as indicated by an arrowed thin solid line in FIG. 4A).Within consecutive overhead, rows 5-9 constitute a so-called lineoverhead which is associated with administration and maintenanceoperations for consecutive transmission spans (defined as a “line”)between transmission apparatuses which are to process multiplexed mainsignals. An line overhead generated in a transmission apparatus istransmitted through transmission spans and regenerator(s), andterminated at a next transmission apparatus (as indicated by an arrowedfat solid line in FIG. 4A). Bytes in the fourth row of the overheadserve as pointers.

In the transmission network illustrated in FIG. 4A, a multiplexer B(2001) and a multiplexer C (2004) are connected by an OC-12 transmissionline through regenerators 2002, 2003. For example, when D bytes referredto as data communication channels and E-byes referred to as orderwiresare to be transmitted and received for administration and maintenanceoperations between the multiplexers B (2001) and C (2004), one of themultiplexers (for example, B) inserts data and speech signals requiredto the maintenance operation into D4-D12 bytes and E2 byte in a lineoverhead and transmits the line overhead onto the OC-12 transmissionline. Since the line overhead is transmitted through the OC-12transmission line and regenerators and terminated at the multiplexer (Cin FIG. 4A) on the reception side, the administration and maintenanceoperations are carried out between the multiplexers B (2001) and C(2004).

However, if the number of subscribers increases or if the amount oftransmitted and received signals is increased, installation ofadditional multiplexers and modifications to the transmission networkmay be required corresponding to the increase in subscribers or signals.FIG. 4B illustrates an example of a modified network, where multiplexerscorresponding to the multiplexers B (2001), C (2004) are additionallyinstalled (for simplicity, one each of the multiplexers B (2001), C(2004) only is illustrated in FIG. 4B), while a plurality of high speedand large-scaled multiplexers E (2008), F (2009) are introduced in placeof the regenerators 2002, 2003 and connected by a high speedtransmission path OC-192 for processing a larger amount of signals at ahigher rate, thus modifying the transmission network.

When the transmission network is modified from the configurationillustrated in FIG. 4A to the configuration illustrated in FIG. 4B dueto an increase in the amount of communications, an overhead from themultiplexer B (2001) is terminated at the multiplexer E (2008) becauseSDH or SONET defines that a line overhead is terminated at eachmultiplexer. If no countermeasures were taken, the modification in thetransmission network would result in lack of the administration andmaintenance operations previously performed between the multiplexers B(2001) and C (2004) using the overhead before the installation of theadditional multiplexers (FIG. 4A).

To solve this problem, the present invention provides each multiplexerwith a function of passing a tributary overhead therethrough, asillustrated in FIG. 4(C). Then, by building a communication network withthe multiplexer of the invention, an overhead can be transmitted andreceived between desired apparatuses within a transmission system,thereby making it possible to improve the administration and maintenanceoperations of the transmission system. In the following, thetransmission apparatus and the transmission system as well as anoverhead transmission/reception method according to the presentinvention will be described in detail in terms of their configurationsand operations.

FIG. 5 is a block configuration diagram illustrating an embodiment of amultiplexer according to the present invention. The multiplexer of thepresent invention accommodates a plurality of tributary multiplexedsignals each including overheads and multiplexed main signals and a highspeed multiplexed signal including overheads and multiplexed mainsignals to perform processing such as termination, replacement, and soon of overheads in each multiplexed signal as well as demultiplexes themultiplexed main signals in the plurality of tributary multiplexedsignals and the multiplexed main signal in the one high speedmultiplexed signal. For example, the multiplexer of the presentinvention accommodates 16 OC-12s (622.08 Mb/s, see FIG. 1) as tributarymultiplexed signals, and performs multiplexing/demultiplexing of mainsignals between the tributary multiplexed signals and the high speedmultiplexed signal OC-192 (9953.28 Mb/s, see FIG. 2) and overheadprocessing as defined by SONET. In addition, the multiplexer passes atributary overhead in a tributary multiplexed signal inputted thereto.It is of course understood that the foregoing is a mere example, andmultiplexed signals to be accommodated may be transmitted at any otherrate than OC-1˜192 or the rate defined by SDH, and the number ofmultiplexed signals to be accommodated may be varied depending on thetype of multiplexed signals to be accommodated.

In FIG. 5, the multiplexer according to the present invention comprisesM sets of tributary signal transmission/reception units 10-1˜10-M forinputting and outputting tributary multiplexed signals to processoverheads and main signals included therein; a set of high speed signaltransmission/reception unit 11 for inputting and outputting a high speedmultiplexed signal to process overheads and main signals includedtherein; a main signal multiplex/demultiplex convertor unit 100 formultiplexing/demultiplexing and converting the main signals in thetributary multiplexed signals and the main signals in the high speedmultiplexed signal; overhead processing units 300 (300-1, 300-2),constituting a feature of the present invention, each forcross-connecting overheads included in each of the tributary multiplexedsignals and the high speed multiplexed signal and for passing thecrossconnected overheads through the multiplexer itself; and a controlunit 400 for controlling the entire multiplexer. The multiplexer iscomposed of the above components to perform multiplexing/demultiplexingand conversion of the signals and overhead processing.

More specifically, each of the tributary signal transmission/receptionunits 10-1˜10-N comprises a SOH (section overhead) termination unit20-1˜20-N for performing the reception of a tributary signal and theprocessing for a received section overhead and for extracting a portionof section overhead bytes which is passed therethrough and transmittedto far end multiplexer; a LOH (line overhead) termination unit 30-1˜30-Nfor processing a received line overhead and for extracting a portion ofline overhead bytes in a manner similar to the section overhead; a LOHinsertion unit 80-1˜80-N for adding transmitted line overhead bytes andfor inserting line overhead bytes transmitted from far end multiplexer;and a SOH insertion unit 90-1˜90-M for adding transmitted sectionoverhead bytes, for inserting section overhead bytes transmitted fromfar end multiplexer, and for transmitting a tributary signal. The highspeed transmission/reception unit 11 comprises a high speed signal SOHtermination unit 60; a LOH termination unit 70; a LOH insertion unit 40;and a SOH insertion unit 50, all of which are similar to thecorresponding units in the tributary signal transmission/reception unit10. Each of the overhead processing units 300-1, 300-2 comprises anoverhead multiplex unit 110, 130 for collecting overheads extracted fromeach multiplexed signal by the transmission/reception unit (10-1˜10-M,11); a space switch unit 200, 210 for crossconnecting collectedoverheads in accordance with predetermined rules in order to transmitthe overheads to far-end multiplexer; and an overhead demultiplex unit120, 140 for distributing the crossconnected overheads to the LOHinsertion units or to the SOH insertion units of the respectivemultiplexed signal transmission/reception units (10-1˜10-M, 11). Withthe configuration mentioned above, the present invention provides amultiplexer which accommodates tributary multiplexed signals and a highspeed multiplexed signal for multiplexing/demultiplexing main signalsincluded therein, passes therethrough certain tributary overheads, whichhave been previously determined in a transmission system using themultiplexers, and processes (terminates/adds) the overheads, so as toenable desired multiplexers to use the overheads therebetween, thusmaking it possible to provide a multiplexer which is superior inadministration and maintenance operation capability as well as achievesa highly usable and flexible system configuration.

In the following, the configuration and operation of the multiplexer andthe operation of the transmission system according to the presentinvention will be described in connection with an example in whichoverheads in 15 multiplexed signals within tributary signals (orderwiresE1, E2, data channels D1-D12, transmission switching control bytes K1,K2, inter-line (apparatus-to-apparatus) error administration byte B2)are passed through a multiplexer which accommodates a high speedmultiplexed signal through OC-192 and tributary multiplexed signalsthrough 16 OC-12s, and used between 15 pairs of transmission apparatuseswhich use the tributary multiplexed signals (between the multiplexers Band C illustrated in FIG. 4B).

FIGS. 6 and 7 are overhead structure diagrams illustrating overheads ina high speed signal used in the multiplexer of this embodiment, intowhich certain overheads in tributary signals are inserted so that theoverheads are passed to far end multiplexer. Specifically, FIG. 6illustrates a general configuration of the overheads and a detailedconfiguration of a portion of the overheads, and FIG. 7 illustrates adetailed configuration of the remaining overheads.

First explained is a multiplexing operation for receiving tributarymultiplexed signals, converting them into a high speed multiplexedsignal, and transmitting the high speed multiplexed signal.

The SOH termination units 20-1˜20-M and the LOH termination units30-1˜30-M of the respective tributary signal transmission/receptionunits 10-1˜10-M perform normal termination processing for overheads ofreceived multiplexed signals in a manner similar to that performed by anormal multiplexer, and send main signals to the main signalmultiplex/demultiplex convertor unit 100. Here, the normal terminationprocessing refers to a check for the normality of a section and a linebetween the multiplexer itself and a far end tributary transmissionapparatus on the tributary multiplexed signal transmission side, whereinthe synchronization of the tributary multiplexed signals is establishedusing A byte, and erroneous signals are checked by B1 byte, by way ofexample. In other words, the processing associated with administrationand maintenance operations of the tributary line or section is performedusing overheads, for example, defined by ANSI T. 105. The main signalsof the tributary multiplexed signals are multiplexed in main signals ofa high speed signal in accordance with multiplexing rules (for example,mapping from FIG. 1 to FIG. 2) previously determined in the main signalmultiplex convertor unit 100. On the other hand, the multiplexer of thepresent invention passes a portion of these tributary overheads for usebetween desired multiplexers, so that the SOH termination units20-1˜20-M and the LOH termination units 30-1˜30-M drop receivedoverheads (except for B2 bytes) as they are so as to transmit theoverheads to the overhead processing unit 300-1. Since B2 bytes cannotbe simply passed for the reason later described, each of the LOHtermination units 30-1˜30-M detects the number of errors in a tributaryline detected as normal B2 bytes termination processing and transmitsthe encoded number of detected errors to the overhead processing unit300-1.

The overhead processing unit 300-1 performs cross-connecting andprocessing for selecting a portion of the tributary overheads andinserting the selected tributary overheads into an undefined area ofoverheads in a high speed multiplexed signal to be transmitted, in orderto pass therethrough the selected portion of the tributary overheadsreceived by the multiplexer to transmit to far end multiplexer. Theoverhead multiplex unit 110 collects overheads transmitted from therespective tributary signal transmission/reception units 10-1˜10-M.Specifically, when tributary multiplexed signals are 16 OC-12s, theoverheads formatted as illustrated in FIG. 1 are sent from therespective tributary signal transmission/reception units 10-1˜10-M, sothat the respective units 10-1˜10-M are sequentially accessed using amultiplexer in the multiplex transmission apparatus to select tributaryoverheads from the respective units 10-1˜10-M, which are to be passed,and the selected overheads are grouped in accordance with the type ofoverheads. As an example, D4 bytes (see FIG. 1) located in the sixthrow, the first column of respective tributary multiplexed signals arecollected and mapped in the high speed multiplexed signal from the fifthcolumn to the 61th column of the sixth row (see 1000, 1002 in FIG. 6),such that the D4 bytes are passed by means of the high speed multiplexedsignal. In this way, overheads of the tributary multiplexed signals aregrouped into respective types and inserted into undefined bytes in highspeed multiplexed bytes, so that the respective units are sequentiallyaccessed to collect each type of overheads. More specifically, in theexample given above, multiplexing is performed such that only the D4bytes in the respective tributary multiplexed signals are collected. Inaddition, the multiplexer in the multiplex transmission apparatus may beprovided with a buffer on the input side thereof for enabling themultiplexing even if the respective tributary multiplexed signals aredifferent in the frame phase. Alternatively, the respective units mayalign the phases of signals to be sent to the overhead multiplex unit110. While the B2 bytes have been described to be received after theprocessing in the LOH termination units, the B2 bytes themselves may bereceived from the LOH termination units so that the above-mentionedprocessing is performed in the overhead multiplex unit 110.

The space switch unit 200 crossconnects the tributary overhead bytestransmitted thereto from the overhead multiplex unit 110 so as tofurther rearrange the tributary overhead bytes. While the space switchunit is used in this embodiment, a time switch may of course be usedinstead. Any switch may be used as long as it can crossconnect thetributary overheads for selection, reordering, and so on, such that theoverheads received from respective tributary multiplexed signals can beinserted into predetermined ones of undefined bytes in a high speedmultiplexed signal. Specifically, in this embodiment, out of 16tributary multiplexed signals on OC-12s, overheads of 15 multiplexedsignals (1-15) are passed, so that the space switch unit 200 selectsonly overheads to be passed within the multiplexed signals (1-15) fromthe output of the overhead multiplex unit 110. The selected overheadsinclude, for example, orderwires E1, E2, data channels D1-D12, automaticprotection switching control bytes K1, K2, and line(multiplexer-to-multiplexer) error checking byte B2 (see 1000-1004,1100-1107, and 1200-1208 in FIGS. 6 and 7). Also, as previouslyillustrated in FIG. 1, D1 byte located nearer to the head of the framethan D4 byte is inserted into the high speed multiplexed signal afterpassing through the multiplexer later than D4 byte (see 1000, 1001, 1004in FIG. 6). In other words, the space switch unit 200 also rearrange theorder of tributary overheads to be passed such that a specified type oftributary overhead can be inserted into a previously specified one ofundefined bytes in a high speed overhead area.

The overhead demultiplex unit 120 receives tributary overhead bytes sentfrom the space switch unit 200, and demultiplexes the received overheadsinto a section overhead and a line overhead using a demultiplexer aswell as demultiplexes the received overheads such that they are insertedinto previously determined bytes in an overhead area of the high speedmultiplexed signal illustrated in FIGS. 6 and 7, and transfers thedemultiplexed overheads to the high speed signal transmission/receptionunit 11. Taking D4 byte of each tributary multiplexed signal as anexample, the D4 byte of each tributary multiplexed signal is separatedfrom each tributary multiplexed signal such that D4 bytes of therespective tributary multiplexed signals are inserted into every fourcolumns of the overhead area in the high speed multiplexed signal fromthe fifth column to the 61st column of the sixth row, as indicated by1000 and 1002 in FIG. 6.

The high speed signal transmission/reception unit 11 generates overheadbytes for use in a section interval and a line interval of the highspeed transmission side, by means of the SOH insertion unit 50 and theLOH insertion unit 40, in a manner similar to a normal multiplexer. Inaddition, the high speed transmission/reception unit 11 inserts theoverhead extracted by the tributary signal reception unit and receivedfrom the overhead demultiplex unit 120 into previously specifiedundefined bytes in a line overhead of a high speed signal, asillustrated in FIGS. 6 and 7, to create overheads to be transmittedwhich are added to main signals of the high speed signal multiplexed bythe main signal multiplex/demultiplex convertor unit 100 to produce ahigh speed multiplexed signal which is then transmitted from the highspeed signal transmission/reception unit 11. It should be noted that inthis embodiment, if a regenerator is located in the middle of atransmission path, a section overhead is terminated at the regenerator,so that overheads extracted by the tributary signal reception unit areinserted only into a line overhead area of the high speed signal. Ofcourse, if there is no regenerator in the middle of a transmission lineso that a section overhead is not terminated up to a destinationtransmission apparatus, undefined bytes in a high speed section overheadarea may be used for inserting the extracted tributary overheadsthereinto. In this case, a larger number of overheads of tributarymultiplexed signals can be passed.

Next explained is the operation on the demultiplexing side forconverting a received high speed multiplexed signal into tributarymultiplexed signals and transmitting the tributary multiplexed signals.

The SOH termination unit 60 and the LOH termination unit 70 of the highspeed transmission/reception unit 11 terminate overheads in a receivedhigh speed multiplexed signal, similarly to a normal demultiplexer, tosend main signals to the main signal multiplex/demultiplex convertorunit 100, in a manner similar to the tributary signaltransmission/reception unit 10. The main signals are demultiplexed bythe main signal multiplex/demultiplex convertor unit 100 into mainsignals of tributary signals. On the other hand, since the receivedoverheads include those transmitted from a far end multiplexer, whichare to be passed for use as tributary overheads, the SOH terminationunit 60 and the LOH termination unit 70 drop the received overheads asthey are, and transmits them to the overhead processing unit 300-2.

The overhead processing unit 300-2 performs cross-connecting andprocessing for selecting overheads to be inserted into predeterminedlocations in overheads of each tributary multiplexed signals in order topass a portion of the received overheads for transmission to anothertributary transmission apparatus. Thus, the overhead processing unit300-2 performs, on the demultiplexing side, similar processing to thatof the overhead processing unit 300-1 on the multiplexing side in thereverse order. Specifically, the overhead multiplex unit 130 collectsoverheads which have passed through from the far end multiplexer andreceived by the high speed signal transmission/reception unit 11, andthe space switch unit 210 crossconnects the overheads sent thereto fromthe overhead multiplex unit 130 so as to again rearrange them. Also, theoverhead demultiplex unit 140 demultiplexes the overheads sent from thespace switch unit 210 into a section overhead and a line overhead, anddemultiplexes the overheads such that they are inserted into thepreviously specified bytes in the overhead area of each tributarymultiplexed signal illustrated in FIG. 1, and transfers thedemultiplexed overheads to the respective tributary signaltransmission/reception units 10-1˜10-M.

Each of the M sets of tributary signal transmission/reception units10-1˜10-M generates overhead bytes for use in a section interval and aline interval of tributary, in a manner similar to a normal multiplexer,by means of the SOH insertion unit 80-1˜80-M and the LOH insertion unit90-1˜90-M, and inserts overheads extracted by the high speed signalreception unit and received from the overhead demultiplex unit 140 intopreviously specified overhead bytes in the line overhead area of thetributary signal, as illustrated in FIG. 1, to create overheads to betransmitted, which are added to tributary main signals demultiplexed bythe main signal multiplex/demultiplex convertor unit 100 to generate atributary multiplexed signal which is then transmitted to the tributarytransmission line.

Next, the processing of passing the B2 bytes in the transmissionapparatus capable of passing received tributary overheads to far endtransmission apparatus, in accordance with the present invention, willbe explained in connection with the multiplexer used in the foregoingembodiment.

FIG. 8 is an operation explanation diagram for explaining a B2-bytecalculation area for detecting transmission errors in a line interval ina transmission apparatus used in conformity to SONET or SDH. Thetransmission apparatus used in conformity to SONET or SDH uses a pointerlocated in the overhead area as illustrated in FIG. 1 (H-bytes on thefourth row) to identify the phase of a multiplexed signal for signalprocessing such as multiplexing/demultiplexing, in order to reduce adelay of the multiplexed signal. Each transmission apparatus onlyupdates the pointer to a frame forming a multiplexed signal for againidentifying the frame, and does not adjust the phase of the frame on anapparatus-by-apparatus basis as does a conventional transmissionapparatus. For this reason, a pointer update causes a deviation betweenthe B2 calculation area of a received multiplexed signal and the B2calculation area of a multiplexed signal to be transmitted, asillustrated in FIG. 8, whereby even if received B2 bytes are passed in amanner similar to other overhead bytes, a transmission apparatusreceiving overheads cannot correctly detect errors which is notified bythe received B2 bytes.

Stated another way, since the B2 bytes cannot be directly passed asmentioned above, the multiplexing side of the multiplexer is configuredsuch that the respective LOH termination units 30-1˜30-M of thetributary signal transmission/reception units read (terminate) the B2bytes to determine the number of detected errors, and pass a signalencoding the number of detected errors (designated by “j” in thefollowing explanation) to notify a destination transmission apparatus ofthe number of errors. Specifically, the B2 bytes of each tributarymultiplexed signal (first to 12th columns on the fifth row in FIG. 1)are terminated to create a signal (indicative of the number of errors j)which is inserted into undefined bytes in the overhead area of a highspeed multiplexed signal (1103 in FIG. 7), and then transmitted in thehigh speed multiplexed signal.

On the demultiplexing side of the multiplexer, the signal indicative ofthe number of errors j cannot be sent to a tributary as it is, in amanner similar to other overhead bytes, because it is different from B2bytes definition, so that the number of errors cannot be detected in thetributary far end apparatus. In addition, it is the number of errorswhich have occurred between a transmission path between desiredtransmission apparatuses (between the transmission apparatuses B and Cin FIG. 4B) that is desired to be transmitted and received. However,since the value of j does not include the number of errors which haveoccurred on the high speed transmission line between the multiplexers,the high speed line erors are also considered. Thus, the demultiplexingside of the multiplexer of this embodiment is configured to perform thefollowing processing in addition to the processing described above inconnection to the processing performed on the B2 bytes.

First, the LOH termination unit 70 of the high speed signaltransmission/reception unit 11 reads (terminate) the B2 bytes fordetecting transmission errors between the high speed multiplexers (theB2 bytes have been defined in the original high speed multiplexed signaland are shown in the first to 192nd columns of the fifth row in thestructure diagrams of FIGS. 2 and 6) to find the number of errors i. TheLOH termination unit 70 also separates the encoded number of detectederrors j from a line overhead (1103 in FIG. 6) of the high speed signal,and calculates the sum k of i and j.

Next, the overhead multiplex unit 130 of the overhead processing unit300-2 multiplexes this k value with other overhead bytes and transmitsthem to the space switch unit 210. The space switch unit 210 rearrangesthe transferred overhead bytes so as to be directed to the respectivetributary signal transmission/reception units. The overhead demultiplexunit 140 demultiplexes the overhead bytes sent from the space switchunit 210 into section overheads and line overheads which are transferredto the M sets of tributary signal transmission/reception units10-1˜10-M.

Then, the LOH insertion units 80-1˜80-M each generate B2 parities forone frame to be transmitted, invert a number of bits corresponding tothe value of k, insert the B2 bytes as an overhead of a tributarysignal, and transmit the overheads onto a tributary transmission line,thereby enabling a tributary far end transmission apparatus to detecterrors. It will be of course appreciated that the operation for the B2bytes on the demultiplexing side may be performed in a manner similar tothat on the multiplexing side, i.e., the calculation of k may beperformed in the overhead multiplex unit 130, or the calculation of kand the inversion of bits may be collectively performed in the LOHinsertion units 80-1˜80-M.

The multiplexer, which is an embodiment of the transmission apparatus ofthe present invention described above, is configured such that, wheninformation on the system configuration is received from a networkmanagement unit or the like, not shown, the control unit 400 determinessettings for the overhead processing unit 300, the multiplexed signaltransmission/reception units 10-1˜10-M and 11, and so on to therebyenable the selection of the type of overheads to be passed through themultiplexer and the selection of the locations of undefined bytes intowhich the overheads are inserted.

FIG. 9 is a network configuration diagram illustrating an example of theconfiguration of a transmission network using the multiplexer of thepresent invention. More specifically, FIG. 9 illustrates in detail theconfiguration of a network in which the foregoing multiplexerillustrated in FIG. 5 is employed as the additionally installedmultiplexers E, F in the network configuration diagram illustrated inFIG. 4B. It should be noted that FIG. 9 illustrates an example in whichoverheads are transmitted from a multiplexer B to a multiplexer C, andthat the illustrated multiplexers according to the present inventiononly include the multiplexing side and the demultiplexing side requiredto the respective multiplexers.

The transmission network employing the multiplexers of the presentinvention comprises a multiplexer B2001 for multiplexing main signals totransmit a tributary multiplexed signal onto a tributary transmissionline 500; a multiplexer E2006 for further multiplexing tributarymultiplexed signals to transmit a high speed multiplexed signal onto ahigh speed signal transmission line 501; a multiplexer unit F2007 fordemultiplexing a high speed multiplexed signal received from the highspeed transmission line 501 to transmit tributary multiplexed signalsonto a tributary signal transmission line 502; and a multiplexer C2004for further demultiplexing the tributary multiplexed signals totributary main signals, wherein main signals processed by themultiplexer B2001 is transmitted to the multiplexer C2004, whileoverheads in tributary multiplexed signals are also transmitted betweenthe multiplexers B2001 and C2004 to perform administration andmaintenance operations for the transmission system.

Explaining in greater details, when the multiplexer B2001 generates atributary multiplexed signal by multiplexing main signals and addingoverheads to the multiplexed main signals, and transmits the tributarymultiplexed signal onto the tributary signal transmission line 500, themultiplexer E2006 terminates the overheads in the tributary multiplexedsignal, multiplexes the main signals, and generates overheads for a highspeed multiplexed signal, while it performs the overhead passingprocessing which constitutes a feature of the present invention. Aspreviously explained with reference to FIGS. 5-7, the passing processingis carried out by the tributary signal SOH termination unit 20 and thetributary signal LOH termination unit 30, the overhead processing unit300-1 comprising the overhead multiplex unit 110, the space switch unit200, and the overhead demultiplex unit 120, and the high speed signalLOH insertion unit 40 and the high speed signal SOH insertion unit 50,by selectively crossconnecting overheads in the tributary multiplexedsignal specified by the control unit 400 to insert the specifiedoverheads into specified locations in an overhead area of the high speedmultiplexed signal (see FIGS. 6 and 7). When specified overheads includethe B2 bytes, the B2-byte processing is also performed as describedabove.

The multiplexer F2007, which has received the high speed multiplexedsignal including the overheads to be passed, from the multiplexer E2006through the high speed transmission line 501, terminates the overheadsin the high speed multiplexed signal, demultiplexes main signals, andgenerates overheads for tributary multiplexed signals, while it performsthe overhead passing processing which constitutes a feature of thepresent invention. This passing processing, as previously explained, isthe reverse processing of the passing processing performed by themultiplexer E2006, wherein overheads for tributary multiplexed signalsincluded in the high speed multiplexed signal and specified by thecontrol unit 400 to be passed are selectively crossconnected andinserted into specified locations in the overhead area of the respectivetributary multiplexed signals. This overhead passing processing isperformed by the high speed signal SOH termination unit 60 and the highspeed signal LOH termination unit 70, the overhead processing unit 300-2comprising the overhead multiplex unit 130, the space switch 210, andthe overhead demultiplex unit 140, and the tributary LOH insertion unit80 and the tributary SOH insertion unit 90. When the specified overheadsinclude the B2 bytes, the B2-byte processing is also performed asdescribed above.

Thus, even in the transmission network in which the multiplexers E, Fhave been additionally inserted between the original multiplexers B andC, the foregoing configuration and operations enable the overheads inthe low passed multiplexed signals as well as main signals to betransmitted between the multiplexers B and C without being terminated atthe additional multiplexer E or F.

The multiplex transmission apparatus of the present invention can passarbitrary overheads in a multiplexed signal with the configuration andoperations described in connection with the foregoing embodiment. Inother words, even if a transmission system as illustrated in FIG. 4A ismodified to a new system as illustrated in FIG. 4B, the multiplexerdescribed in the foregoing embodiment, if employed for newly addedmultiplexers, passes overheads so far used between the multiplexers Band C without terminating them, so that the overheads can be used alsobetween the multiplexers B and C in the modified system. Thus, theadministration and maintenance operation capability between themultiplexers is not affected by the modification to the configuration ofthe transmission system. In addition, since the multiplexer of thepresent invention permits overheads to be used between any desiredmultiplexers, it is possible to provide a multiplexer which has higheradministration and maintenance operation capability, offers auser-friendly operability, and supports to build a flexible transmissionsystem.

Next, another embodiment of the multiplexer according to the presentinvention and an embodiment of a transmission system or a network usingthe multiplexer will hereinafter be described in detail with referenceto the accompanying drawings.

FIG. 10 is a block configuration diagram illustrating an embodiment ofan add drop multiplexer (hereinafter abbreviated as “ADM”) which is atransmission apparatus according to the present invention. FIGS. 11A and11B are network configuration diagrams illustrating examples of theconfiguration of transmission networks using the ADM of the presentinvention.

The ADM of the present invention accommodates a plurality of tributarymultiplexed signals comprising overheads and main signals multiplexedtherewith and two high speed multiplexed signals comprising overheadsand main signals multiplexed therewith, and performs the processing suchas termination, replacement, and so on for the overheads in therespective multiplexed signals. In addition, the ADM inserts a pluralityof tributary multiplexed main signals into high speed multiplexed mainsignals (add), branches a plurality of tributary multiplexed mainsignals from high speed multiplexed main signals (drop), crossconnectshigh speed multiplexed main signals with each other (cross-connect), andpasses the high speed multiplexed main signals (through). Similarly tothe aforementioned embodiment, the tributary multiplexed signals aretransmitted through OC-12s, while the high speed multiplexed signals aretransmitted through OC-192s. The ADM performs the processing for themain signal and the overhead processing as defined in SONET, and alsopasses a portion of overheads in the multiplexed signals inputtedthereto so that the passed overheads are used by a far end multiplexer.Then, a network employing the ADMs has the ADMs connected to each otherthrough high speed transmission spans (OC-192) in a linear configuration(FIG. 11A) or in a ring configuration (FIG. 11B). In addition,transmission apparatuses such as multiplexers are connected to the ADMsthrough tributary transmission spans (OC-12). With the configurationdescribed above, the ADMs perform the overhead passing processing inaccordance with the present invention to transmit and receive arbitraryoverheads in the tributary multiplexed signals between arbitrarymultiplexers, in addition to the main signal processing as mentionedabove to transmit and receive the main signals between the multiplexers,thereby building a highly flexible transmission system which allows themultiplexers within the network to freely transmit and receive theoverheads as well as the main signals therebetween, and provides ahigher administration and maintenance operation capability.

The configuration of the ADM is substantially the same as that of theaforementioned multiplexer except for a main signal insertion/separationunit 105 additionally provided for performing add, drop, cross-connect,and through operations for the main signals. The remaining functionalblocks used in FIG. 10 are the same as corresponding ones in theaforementioned multiplexer (in FIG. 10, the same functional blocks asthose in FIG. 6 are designated the same reference numerals), except forthe location and number thereof which are modified to be adapted to theADM. In the following explanation, only the configuration and operationsdifferent from the aforementioned multiplexer will be described inparticular.

Since a high speed transmission/reception unit 11 is used to connect theADMs with each other through a high speed transmission span (see FIGS.11A, 11B) in this embodiment, two high speed transmission/receptionunits 11, i.e., a west side unit 11-1 and an east side unit 11-2, areprovided in each ADM so as to be connected to ADMs on both sides. Also,a main signal insertion/separation unit 105 is added between the highspeed signal transmission/reception units 11-1, 11-2 and a main signalmultiplex/demultiplex convertor unit 100 so that the high speed signaltransmission/reception units 11-1, 11-2 can be connected to tributarysignal transmission/reception units 10-1˜10-M, and the high speed signaltransmission/reception units 11-1, 11-2 can be connected to each otherin order to perform the main signal processing as mentioned above.

Further, the ADM is also adapted to pass overheads from a high speedmultiplexed signal to another such that a space switch unit 200 of anoverhead processing unit 300 selectively crossconnects overheads to bepassed, which have been received from one of the high speed signaltransmission/reception units (for example, the East side unit) throughan overhead multiplex unit 130, and transmits the crossconnectedoverheads to the other high speed transmission/reception unit (forexample, the WEST side unit) which inserts the overheads to be passedinto specified bytes in an overhead area of a high speed multiplexedsignal.

Furthermore, when overheads to be passed from a high speed multiplexedsignal to another includes information on the B2 bytes, a LOH insertionunit 40 of the high speed signal transmission/reception unit 11 does notperform the bit inversion of the B2 bytes as does the LOH insertion unit80 of the tributary signal transmission/reception unit 10 in theaforementioned multiplexer, and instead adds the number of errors i (theresult of termination of the B2 bytes), which have occurred on anincoming high speed transmission line, to a received number of errors jto derive a total number of errors k which is transmitted as it is. Inthe alternative, the configuration for detecting transmission errors inthis overhead transmission/reception interval may be implemented by aconfiguration utilizing a tandem connection, later described.

The ADM is also configured to rely on a control unit 400 which indicatesto each functional block the types and locations to be inserted foroverheads to be passed, such that each functional block responsivelyperforms the crossconnecting, selection, and insertion of the overheads.The control unit 400, in turn, is coupled to receive the controlinformation from a network management unit 2017 illustrated in FIGS. 11Aand 11B. Specifically, the network management unit 2017 appropriatelyindicates to each ADM the types and locations to be inserted foroverheads to be passed, so as to prevent conflicts or contradiction,thereby allowing overheads as well as main signals to be freelytransmitted and received between multiplexers within the network.

As described above, according to the present invention, overheads can bepassed through arbitrary multiplexers so that they may be used betweenarbitrary multiplexers likewise in the ADM described in the foregoingembodiment as well as in a transmission network or a transmission systemwhich employs the ADMs. It is therefore possible to provide atransmission system which is free from a change in administration andmaintenance operation capability due to a modification to a systemconfiguration, i.e., which is superior in improved administration andmaintenance operation capability, flexible, and highly usable.

Next explained in the following is a configuration for detectingtransmission errors occurring in a transmission interval betweenmultiplexers which transmits and receives overheads, with reference toanother embodiment different in configuration from the foregoingembodiments. Specifically, this embodiment is equivalent to aconfiguration of a transmission system adopting SONET or SDH whichnotifies transmission errors occurring in an interval from an overheadtransmitting multiplexer to an overhead receiving multiplexer using atandem connection standardized by ANSI or ITU-T, i.e., a configurationwhich passes the B2 bytes through intervening multiplexers. Thus, in theaforementioned multiplexer and ADM, any configuration may be utilized.

FIGS. 12A and 12B are operation explanation diagrams for explaining aconfiguration for detecting transmission errors using a tandemconnection. More specifically, FIG. 12A illustrates the configurationaccording to this embodiment, and FIG. 12B illustrates the configurationfor detecting transmission errors which has been explained in connectionwith the aforementioned multiplexer or ADM. FIG. 13 is a networkconfiguration diagram illustrating the configuration of a transmissionnetwork using a tandem connection. With reference to these drawings, theconfiguration for detecting transmission errors using a tandemconnection according to the present invention will be explained below incomparison with the embodiments described above.

As illustrated in FIG. 12B, the previously described embodiments areconfigured to pass the B2 bytes through a certain multiplexer such thaterrors are detected using the B2 bytes in a transmission intervalbetween different multiplexers. For this purpose, each multiplexer whichpasses the B2 bytes terminates the B2 bytes, detects the number oferrors i, adds the number of errors i to the number of errors j notifiedfrom a source multiplexer to derive a total number of errors k, andtransmits the total number of errors k. Then, the last multiplexer whichpasses the overheads, generates B2 bytes from the final total number oferrors (k′ in FIG. 12B), which have been added in the respectiveintervening multiplexers, and inverts bits of the B2 bytes to notifytransmission errors which have occurred during the transmission from anoverhead originating multiplexer to a destination multiplexer.

In a transmission system adopting SONET or SDH, the management of a pathis basically carried out between a source unit and a destination unit.However, if a path from a source Path Terminating Equipment (PTE) 4000-1to a destination PTE 4000-2 passes through network regions which aremanaged by different management schemes as illustrated in FIG. 13 (inthis embodiment, the path passes through network management regions1-3), it is necessary to independently manage the path in eachmanagement region to locate a region in which a failure has occurred.Thus, as stipulated in ANSI standard T1. 150 or ITU-T standard G. 707,an interval between both boundaries of a management region including aplurality of continuously connected Line Terminating Equipments (LTEs)and lines is defined as a tandem connection for which a managementmethod is defined. Specifically, one of a plurality of path overheadsPOHs included in a payload illustrated in the frame structure diagram ofFIG. 1 is used. Transmission errors are managed using B3 byte in thethird row for detecting errors in a path and Z5 byte in the ninth rowwhich is a byte for managing a tandem connection. Explaining, as anexample, the detection of transmission errors in the tandem connectionin the management region 1, an LTE 4001 monitors the B3 byte for thenumber of errors l which have occurred on a path from the source PTE4000-1 to the LTE 4001, and inserts the number of errors l into the Z5byte and transmits the multiplexed signal to a LTE 4002. The LTE 4002again monitors the B3 byte for the number of errors l′ which haveoccurred on a path from the source PTE 4000-1 to the LTE 4002, andsubtracts the number of errors l received through the Z5 byte from thenumber of errors l′ to manage the number of errors which have occurredin the tandem connection.

In the configuration described in this embodiment for detectingtransmission errors occurring in a transmission interval betweenmultiplexers which transmits and receives overheads, the firstmultiplexer 2030 which first passes overheads in a tributary multiplexedsignal is regarded as an entrance multiplexer of the tandem connection,and the last multiplexer 2032 which passes the overheads in thetributary multiplexed signal is regarded as a terminal multiplexer, asillustrated in FIG. 12A, so that the number of errors is communicatedutilizing a tandem connection management method using the B3 byte andthe Z5 byte as mentioned above.

Specifically, the multiplexer 2030 encodes the number of errors (j)detected after terminating the B2 bytes of the tributary multiplexedsignal, inserts the encoded number of errors into a specified byte in anoverhead area of a high speed multiplexed signal, and transmits the highspeed multiplexed signal onto a transmission line to the destinationmultiplexer (3001, 3002), in a manner similar to the aforementionedembodiments. Also, the multiplexer 2030 checks the B3 byte for thenumber of errors which have occurred on a path up to the multiplexer2030 itself, and inserts the detected number of errors (l) into the Z5byte to transmit the number of errors (3003).

An intervening multiplexer 2031 which passes overheads therethrough onlypasses the received number of errors (3004, 3002), and does not add thenumber of errors (i), detected on a line between the multiplexers andnotified through the B2 bytes, to the number of errors (l), as does themultiplexers of the aforementioned embodiment (see FIG. 12B). Of course,the termination of the B2 bytes (error detection on the reception sideand generation of parities on the transmission side) is performed asusual (in conformity to the standard), which, however, is processedindependently of the overhead passing processing explained in thisembodiment. Also, since the B3 byte and the Z5 byte may be checkedbetween paths, that is, on the root between the Path TerminatingEquipments (PTEs) 4000-1 and 4000-2 of FIG. 13, the transmissionapparatus of this embodiment does not require other processing (checkfor the number of errors) except for passing these bytes as the mainsignals.

The multiplexer 2032 extracts the number of errors (j) transmitted fromthe multiplexer 2030 (3004). Also, the B3 byte is checked as explainedabove, and the value of the received Z5 byte (l) is subtracted from thenumber of errors (l′) occurring on the path up to the multiplexer 2032to derive the number of transmission errors (l′-l, i.e., the number oferrors equal to i+i′ in FIG. 12B) which has occurred on a transmissionpath between the multiplexers 2030 and 2032 (tandem connection) (3005).Then, the received number of errors (j) and the calculation result(l′-l) are added to derive the number of errors (k′) which have occurredon a transmission interval from the source multiplexer to themultiplexer 2032 in a manner similar to the aforementioned embodiment(3001), and the number of bits equal to k′ in generated B2 parity bitsare inverted and transmitted to a destination multiplexer 2033 (3006).

The configuration and method as described above also provide a resultsimilar to the aforementioned embodiment in which the B2 bytes arepassed through intervening multiplexers, thus making it possible tonotify a destination apparatus of transmission errors which haveoccurred on a transmission interval between the source and destinationmultiplexers which transmit and receive the overheads. Also, accordingto the configuration described above, an intervening multiplex whichpasses overheads therethrough need not perform calculations. Since thefirst and last multiplexers are only required to perform calculations,the amount of hardware can be reduced in an interval including a largenumber of multiplexers which pass overheads therethrough. One of theaforementioned configuration (FIG. 12B) and the configuration of thisembodiment (FIG. 12A) may be selected depending on the scale of aparticular network or system.

As described above, since a transmission network is built with themultiplexers and ADMs provided with the function of passing overheads,in accordance with the present invention, a modification to the networkconfiguration will not affect the administration and maintenanceoperation capability as has been often the case of the conventionaltransmission system. Also, since arbitrary overheads can be transmittedand received between arbitrary multiplexers within a transmissionnetwork, a flexible network having a superior administration andmaintenance operation capability can be readily realized. Specifically,the orderwires E1, E2 may be transmitted and received between arbitrarymultiplexers to allow a craft man to make a speech communication. Inaddition, the data communication channels D1-D12 may be transmitted andreceived to set a variety of parameters and so on for multiplexers, thusmaking it possible to build a flexible transmission network which can bereadily modified in configuration. Particularly, in accordance withSONET, the D1-D12 are transmitted from a network management unit or thelike to make settings for respective multiplexers, in which case if theconfiguration of passing such communication channels D1-D12 throughintervening multiplexers is employed according to the present invention,data required for settings can be readily sent to a destinationmultiplexers without the need for the intervention of complicatedprocessing performed by a control unit of a multiplexer, as is the caseof the conventional multiplexers, each of which would have to onceterminate and again transmit overheads. It is therefore appreciated thatthe present invention is extremely effective in the maintenanceoperation management of a transmission network. Further, since theswitching control bytes K1, K2 for controlling the switching oftransmission lines may be transmitted and received to achieve theselection of a transmission line free from contradiction betweenmultiplexers, the network configuration can be promptly modified orreconfigured if a transmission line fails. This is also effective in themaintenance operation management for the transmission network. Fortransmission errors, although the B2 bytes cannot be directlytransferred between source and destination multiplexers, the number ofoccurring errors is reliably detected and notified, so that it ispossible to realize the management of the error ratio in a transmissioninterval equivalent to that achieved by the B2 bytes which aretransmitted and received between directly coupled source and destinationmultiplexers without any intervening multiplexer. It will be of courseappreciated that if other overheads illustrated in FIG. 3 are alsotransferred in a manner similar to the overheads explained above, theycan be utilized likewise for the administration and maintenanceoperation between multiplexers.

In the present invention, undefined bytes in an overhead area of amultiplexed signal defined by the standard as mentioned above areselected, and overheads to be passed through a transmission apparatusare inserted into the selected undefined bytes in the multiplexedsignal, and then transmitted in the multiplexed signal. In other words,in a transmission network or a transmission system, bytes to be used maybe previously determined before transmission and reception, or thepreviously explained data communication channels may be used to modifysettings of used bytes before transmitting and receiving these bytes.Since free settings can be made depending on the number of transmissionapparatuses and the amount of administration and maintenance operationinformation within a transmission network as long as undefined bytes areavailable, it is possible to provide a transmission apparatus and atransmission network which have an administration and maintenanceoperation capability high enough to flexibly cope with modifications innetwork configuration or administration and maintenance operation methodor with future modifications in the standards, if any.

According to the multiplex transmission apparatus and the multiplextransmission network of the present invention, desired transmissionapparatuses within a network are allowed to transmit and receiveoverheads for carrying information associated with administration andmaintenance operations, which have been terminated at each transmissionapparatus in a conventional transmission system, so that it is possibleto provide a transmission apparatus and a transmission network whichhave a high administration and maintenance operation capabilityindependent of modifications in network configuration or of an employedadministration and maintenance operation method.

1. An optical communication system for transmitting optical signalstransmitted on a plurality of first optical transmission lines to aplurality of third optical transmission lines via a second opticaltransmission line, comprising: a first optical transmission unitconnected to said first and second optical transmission lines, forreceiving first optical signals transmitted on said first opticaltransmission lines and transmitting a second optical signal obtainedfrom said first optical signals to said second optical transmissionlines; and a second optical transmission unit connected to said secondand third optical transmission lines, for receiving said second opticalsignal from said second optical transmission line and transmitting thirdoptical signals obtained from said second optical signal to said thirdoptical transmission lines, wherein said first optical transmission unitextracts optional information included in first regions of a pluralityof first overheads of said first optical signals and enters saidoptional information extracted into a second region which is not definedfor a particular use in a second overhead of said second optical signaland transmits said second optical signal comprising said optionalinformation in said second overhead to said second transmission lines;and said second optical transmission unit extracts said optionalinformation in said second overhead of said second optical signal andenters said optional information extracted into third regions of aplurality of overheads of third optical signals and transmits said thirdoptical signals comprising said plurality of optional information insaid third overheads to said third transmission lines, wherein saidfirst regions are predetermined for storing said optional information insaid first overheads, and said third regions are predetermined forstoring said optional information in said third overheads.
 2. Theoptical communication system according to claim 1, wherein said secondoptical transmission line transmits said second optical signal fasterthan said first and third optical transmission lines, and said firstoptical transmission unit multiplexes said first optical signals toobtain said second optical signal, and said second optical transmissionunit demultiplexes said second optical signal to obtain said thirdoptical signals.
 3. The optical communication system according to claim2, wherein said first optical transmission unit multiplexes saidoptional information and enters said optional information multiplexedinto said region which is not occupied for use in said second overheadof said second optical signal multiplexed, and said second opticaltransmission unit demultiplexes said optional information multiplexedand enters said optional information demultiplexed into said pluralityof said regions allotted for said optional information in said pluralityof third overheads of said third optical signals demultiplexed.
 4. Theoptical communication system according to claim 1, further comprising: asystem management unit connected to said first and second opticaltransmission units, for indicating said optional information to beextracted and said region which is not occupied for use in said secondoverhead to said first and second optical transmission units.
 5. Theoptical communication system according to claim 1, wherein said firstoverheads and said third overheads have the same format, and said firstregions and said third regions are the same regions of said first andthird overheads, respectively.
 6. An optical communication system fortransmitting optical signals transmitted on a plurality of first opticaltransmission lines to a plurality of third optical transmission linesvia at least one second optical transmission lines, comprising: a firstoptical transmission unit connected to said first and second opticaltransmission lines; and a second optical transmission unit connected tosaid second and third optical transmission lines; wherein said firstoptical transmission unit extracts first information concerning a numberof errors included in first regions of a plurality of first overheads offirst optical signals received from said first optical transmissionlines and adds a number of errors detected from first optical signals tosaid number of errors of said first information to obtain secondinformation concerning a number of errors and enters said secondinformation into a second region which is not defined for a particularuse in a second overhead of a second optical signal and transmits saidsecond optical signal comprising said second information in said secondoverhead to said second optical transmission line, and said secondoptical transmission unit extracts said second information in saidsecond overhead of said second optical signal received from said secondoptical transmission line and adds a number of errors of said secondinformation to obtain third information concerning a number of errors,and enters said third information into third regions of a plurality ofthird overheads of third optical signals and transmits said thirdoptical signals comprising said third information in said thirdoverheads to said third transmission lines, wherein said first regionsare predetermined for storing said optional information in said firstoverheads, and said third regions are predetermined for storing saidoptional information in said third overheads.
 7. The opticalcommunication system according to claim 6, wherein said second opticaltransmission line transmits said second optical signal faster than saidfirst and third optical transmission lines, and said first opticaltransmission unit multiplexes said first optical signals to obtain saidsecond optical signal, and said second optical transmission unitdemultiplexes said second optical signal to obtain said third opticalsignals.
 8. The optical communication system according to claim 7,wherein said first optical transmission unit multiplexes said firstinformation to obtain said second information and enters said secondinformation multiplexed into said region which is not occupied for usein said second overhead of said second optical signal multiplexed, andsaid second optical transmission unit demultiplexes said secondinformation multiplexed to obtain said third information and enters saidthird information demultiplexed into said plurality of regions allottedfor said third information in said plurality of third overheads of saidthird optical signals demultiplexed.
 9. The optical communication systemaccording to claim 6, further comprising: a system management unitconnected to said first and second optical transmission units, forindicating information concerning a number of errors and said regionwhich is not occupied for use in said second overhead to said first andsecond optical transmission units.
 10. The optical communication systemaccording to claim 6, wherein said first overheads and said thirdoverheads have the same format, and said first regions and said thirdregions are the same regions of said first and third overheads,respectively.
 11. An optical communication system for transmittingoptical signals transmitted on a plurality of first optical transmissionlines to a plurality of third optical transmission lines via at leastone second optical transmission line, comprising: a first opticaltransmission unit comprising: a plurality of first optical signalreceiving portions connected to each of said first optical transmissionlines, for receiving a plurality of first optical signals from saidfirst optical transmission lines and extracting specified informationincluded in first regions of a plurality of first overheads of saidfirst optical signals; a first overhead processing portion connected tosaid plurality of first optical signal receiving portions, for arrangingsaid specified information extracted; and a first optical signaltransmission portion connected to said first overhead processingportion, for entering said specified information arranged by said firstoverhead processing portion into a second region which is not definedfor a particular use in a second overhead of a second optical signal andfor transmitting said second optical signal comprising said specifiedinformation in said second overhead to said second optical transmissionlines, and a second optical transmission unit comprising: a secondoptical signal receiving portion connected to said second opticaltransmission line, for receiving said second optical signal from saidsecond optical transmission line and extracting said specifiedinformation in said second overhead; a second overhead processingportion connected to said second optical signal receiving portion, forarranging said specified information extracted; and a plurality ofsecond optical signal transmission portions connected to said secondoverhead processing portion, for entering said specified informationarranged by said second overhead processing portion into third regionsof a plurality of third overheads of a plurality of third opticalsignals for transmitting said third optical signals comprising saidspecified information in said third overheads to said third opticaltransmission lines, wherein said first regions are predetermined forstoring said optional information in said first overheads, and saidthird regions are predetermined for storing said optional information insaid third overheads.
 12. The optical transmission system according toclaim 11, wherein said first overhead processing portion arranges saidspecified information according to kinds of information, said secondoverhead processing portion arranges said specified information so thatsaid specified information are entered into said regions allotted forsaid specified information of said third overheads.
 13. The opticaltransmission system according to claim 11, wherein said first opticalsignal transmission unit comprises a first control portion forindicating said specified information to be extracted to said firstoptical signal receiving portions, indicating a method of arranging saidspecified information to said first overhead processing portion andindicating said region which is not occupied for use in said secondoverhead to said first optical signal transmission portion, and saidsecond optical signal transmission unit comprises a second controlportion for indicating said region which is not occupied for use of saidsecond overhead to said second optical signal receiving portion and amethod of arranging said specified information to said second overheadprocessing portion.
 14. The optical transmission system according toclaim 13, further comprising a system management unit connected to saidfirst and second optical signal transmission units, for indicating saidspecified information to be extracted and said region which is notoccupied for use in said second overhead to said first and secondcontrol portions.
 15. The optical communication system according toclaim 11, wherein said first overheads and said third overheads have thesame format, and said first regions and said third regions are the sameregions of said first and third overheads, respectively.